(1) Field of the Invention
The present invention relates to the exercise of control over a gas turbine engine and particularly to the use of a free turbine engine employed in a helicopter rotor drive system as a means for damping oscillations in the drive system. More specifically, this invention is directed to apparatus for controlling the delivery of fuel to the gas generator of a turboshaft propulsion system and especially to a fuel control for rotary wing aircraft wherein fuel flow is varied as a function of speed oscillations of the rotor drive train. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
The typical helicopter rotor drive train consists of a turboshaft engine comprising a gas generator and free turbines, the turbines being mechanically coupled to the main and tail rotors of the vehicle. This drive train is a highly resonant system with primary modes, which are separate for the main and tail rotors, in the 2 to 8Hz frequency range. While some damping of this system results from the aerodynamic drag on the rotor blades, gear reductions and other inherent losses in the system, considerable additional damping is nevertheless highly desirable. It has previously been difficult to provide additional active damping in a typical rigid rotor system. In a fully articulated rotor system, where the individual main rotor blades are supported such that they may undergo limited movement in two directions with respect to the rotor hub, the damping may be increased through the use of viscous lag dampers installed between each blade and the hub. These hydro-mechanical lag dampers are, however, subject to possible failure due, by way of example only, to damage when the vehicle is operated in a combat zone.
Continuing with the above discussion, when a helicopter is exposed to a wind gust or there is a sudden change in collective pitch commensurate with a commanded maneuver, the rotor drive train may "ring", i.e., go into oscillation. The frequency of the load change, i.e., the wind gust or collective pitch variation, may be such that the magnitude of the oscillation will be substantial. It is to be noted that this oscillation is, in part, possible because the mechanical drive train, which includes a free turbine, is decoupled from the gas generator.
It is to be noted that the stability of the engine and rotor system as a whole can be maintained. Thus, the sensed free turbine output shaft speed is customarily employed as a control parameter in the closed loop system. However, damping of the rotor drive train is not enhanced by presently available closed loop controls since, in order to maintain closed loop stability, existing gas generator fuel control technology calls for the filtering of the resonant frequencies from the free turbine speed feedback signal. This removal of transient signals from the free turbine speed input signal to the fuel control is in the interest of insuring that the gas generator will not be operated in a mode which would actually excite the resonant rotor drive train. It should, incidently, be noted that present fuel controls actually reduce to some extent any damping of the mechanical drive system since the filtering does not remove all short term free turbine speed variations from the feedback signal commensurate with free turbine speed.